Light barriers are known in the workplace safety sector.
Generally, they emit one or more light rays, which are generally referred to as light grid or light barrier.
Some types of light barrier are utilized to detect movement or the intrusion of a foreign body within a zone, such as for example the operating zone of a machinery, and can guarantee the protection of the operators working with the machinery or other industrial equipment. Other light barriers are implemented in order to control automated industrial processes and are used to verify the assembly process by counting the objects, recognizing the products being transported for example on a conveyor belt, recognizing irregular shapes, etc. Numerous other applications are possible. A non-exhaustive list includes for example:                Automated machines;        Packaging, goods handling and warehousing machines;        Textile, wood, and ceramic processing machines;        Automatic and semi-automatic assembly lines;        Automated warehouses.        
In general, light barriers include two optical units, often called “bars”; one of the two units is the emitter while the other is the receiver. The two optical units can be coupled together; that is to say the same bar can include both the emitter and the receiver. For example, the same optical unit can include light emitters and light receivers.
Light barriers generally utilize visible spectrum or infrared light sources as the emitter. For example, said sources can include light emitting diodes (LED) mounted at a distance from each other along a transmitter bar that can be placed for example on one side of the monitored zone; photo-transistors, photo-diodes, or other photo-receivers can be used as the receiver elements, which can be mounted for example along a receiver bar on the opposite side of the monitored zone.
In a known example of operation of a light barrier represented in FIG. 1, the light sources in emitter bar TX emit beams of light towards the photo-receivers of receiver bar RX. In case one or more beams of light are blocked by an opaque object, such as for example the arm of an operator (indicated with a circle in FIG. 1), a control circuit—not illustrated—blocks the operating machine assigned to the worker, preventing the machine from proceeding with the work or protecting in some other manner the zone and the operator.
As stated above, this interruption can also be utilized to count objects or verify the entrance of goods into a particular zone.
The bars that comprise the optical units, that is to say the bars housing the light emitters and/or receivers, are generally manufactured in aluminum, since this material is easy to work using extrusion or wire drawing. Aluminum is a light, cheap, and easy to work material, and is hence employed to form the bars of the desired length and shape, such that light bars of different shapes and structures can be manufactured while maintaining the manufacturing costs under control, relatively speaking.
However particular sectors, such as for example the pharmaceutical and food processing sectors, require the use of bars comprising the optical units of the light barriers manufactured using a material that is more resistant than aluminum. In fact, in the presence of aggressive chemical agents, such as detergents, which are often used in these environments, aluminum can be easily damaged and does not meet the resistance standards required in these particular technical sectors.
In order to satisfy this requirement, a known method is to encapsulate the aluminum optical modules in tubular elements manufactured with a transparent material, such as for example glass tubes. The tubular element that contains the aluminum bar is then closed by means of metal elements, preferably hermetically in order to always meet the seal requirements in these special technical sectors.
However, the presence of these tubes manufactured with a transparent material implies first of all manufacturing another element in addition to the aluminum bar, increasing the cost and complexity of the assembly, as well as constraining the shape of the optical units, since large variations in the shape would imply an excessive number of different types of tube. Furthermore, glass tubes are extremely fragile to impacts and/or falls.
The solution to manufacture the bar for the optical unit with a material other than aluminum, which can meet by itself—without the use of the external tubes—the previously stated requirements, is generally rejected in the sector because materials such as stainless steel which would guarantee an adequate level of resistance against chemical agents in particular, are considered to be too expensive in the sector, in particular as a result of the time and cost required for their production. Therefore manufacturing a metal tube body starting from a stainless steel bar was considered in the sector to be an excessively expensive solution that would lead to manufacturing a light barrier with a cost that was outside the normally accepted range in the reference market.
Furthermore, particular sectors, such as for example the above mentioned pharmaceutical and food processing sectors, require that the bars comprising the optical units for the light barriers be provided with a seal and that they also be particularly easy to clean. In fact, these sectors employ liquids which are often corrosive, and which, if they find their way inside the bar, may damage the optics present within the bars. Additionally, in order to meet the sanitary regulations required in these particular technical sectors, it is preferable that no recesses or grooves be present in the external surface of the bars because they are difficult to clean and can become a bacterial growth site.
Typical bars according to the known art are illustrated for example in FIG. 17 (only the end of the bar is shown), where a plurality of locations for the screws is shown, which are the potentially difficult to clean zones. The screws are used to close an opening of the bar with a “plug”, where the optics are inserted into the bar itself through said opening.
In order to meet this requirement, the bars in the prior art are provided with a closed tubular element where necessary in order to plug the different types of openings by means of plastic or metal elements, which preferably provide a seal in order to meet the seal requirements in these special technical sectors, and which are glued to the tubular element. The glue provides the possibility of avoiding mechanical components with recesses or grooves such as those in FIG. 17. However, the use of glue means the bars can no longer be opened in order to access the optics housed inside, or are otherwise difficult to open.